The present invention relates to a rolling bearing which is incorporated into steelworking facilities (facilities for processing steel into products) or paper machine facilities and which is to be used under harsh conditions such as high contact pressure or high temperatures.
The present invention also relates to a rolling bearing for use in railcars such as the Shinkansen (bullet train), a narrow-gauge line, or a freight train.
The present invention further relates to a rolling bearing which is to be used in, e.g., machine tool facilities, for a long period of time at high speed rotation and is subject to high-cycle rolling contact fatigue.
More particularly, the present invention relates to a rolling bearing which is resistant to cracking or flaking, which would otherwise shorten the life of the bearing, and which is ensured of having a stable and prolonged life.
Moreover, the present invention also relates to an ultrasonic detection and inspection method suitable for use in inspecting imperfections in a bearing raceway ring of the rolling bearing, such as presence of non-metallic intevening material in a position immediately below a bearing raceway surface of the bearing raceway ring.
Presence of non-metallic intevening material in a position immediately below a bearing raceway surface of a bearing raceway ring has hitherto been known to greatly affect the life of a rolling bearing. For this reason, there has conventionally been adopted a method of extending the life of a bearing by limiting the amount of non-metallic intevening material in steel during a phase of producing steel material for bearing. As described in, e.g., Japanese Patent Application Laid-Open (i.e., Japanese Patent Unexamined Publication) Nos. 145883/1994, 56640/1991, 117804/1993, and 192790/1994, there has already been proposed a technique aimed at prolonging the life of a bearing. Taught by the technique is that the bearing life can be prolonged by specifying the number of pieces of oxide-based intervening material (imperfections) consisting primarily of Al2O3 or the number of pieces of Ti-based intervening material (imperfections) consisting primarily of TiN, each existing in a given area of subject steel.
Since a rolling bearing employed in paper machine facilities is used with an inner ring thereof being subjected to fitting stress, hoop stress is exerted on a bearing inner ring in a high-temperature environment, which raises a problem of cracking originating from large pieces of non-metallic intervening material located in the vicinity of an inner-diameter surface. In order to address the problem, there has been adopted a measure for preventing occurrence of cracking, by means of carburization of a bearing surface to there by impart compressive stress to the surface. As described in, e.g., Japanese Patent Application Laid-Open No. 307457/1994, there has already been disclosed a technique of preventing occurrence of hoop stress cracking in an inner ring having fitting stress exerted thereon, which would otherwise be caused in a high-temperature environment, by means of carbonitriding medium carbon steel for preventing occurrence of cracking.
In the case of a bearing for use in a railcar typified by the Shinkansen, in which the bearing is continuously used over a long period of time at high speed, one can predict that the bearing is subject to high-cycle rolling contact fatigue and that a serious accident will arise if the bearing is exfoliated and short-lived. With a view toward reducing the amount of non-metal intervening material in steel, steel having a limited amount of oxygen has been used as bearing material in such an application, or steel having a reduced amount of intervening material existing in a given area of subject steel has been used as bearing material, as described in connection with the related art.
Ultrasonic flaw detection is employed for detecting flaws in steel used in a bearing. With the objective of detecting flaws, such as macro-streak-flaws or imperfections caused by a hole not having been crimped, a steel manufacturer inspects all steel bars and all cross sections thereof, through ultrasonic flaw detection. As a result of removal of the imperfections that have been found through inspection, large imperfections in steel material used for a bearing have been eliminated. As the flaw detection method, there has already been known a normal beam method (see xe2x80x9cSpecial Steelxe2x80x9d Vol. 46, No. 46, pg. 31), wherein flaw detection is effected by causing ultrasonic waves to propagate from an outer circumferential surface of rolled steel to the inside thereof, in water or on a table.
However, flaws that can be detected in the rolled steel by ultrasonic flaw detection measure several millimeters in length. In-some instances, flaking or cracking has originated in actually-used bearings, within a short period of time, from large non-metal intervening material of hundreds of micrometers or from an aggregate into which small simple substances have coagulated. Highly accurate detection of flaws of such sizes has been impossible for two reasons. Namely, one of which is that flaws are detected at high speed through inspection during the steelworking process in order to improve productivity, and the other of which is that a steel product is inspected while remaining in a rolled state and having rough crystal grains therein and a rough surface layer. Accordingly, large noise resultantly arises during the course of detection of flaws.
Presence of large non-metallic intervening material has been well known to exert a great influence upon extending the life of a rolling bearing. If presence of such large non-metallic intervening material can be detected beforehand at the time of manufacture of a bearing, an extension of the life of a manufactured bearing can be expected. A steel product to be used for a bearing is subjected to ultrasonic flaw detection in a billet phase during a steel working process or a rolled-round bar phase. However, a flaw detection frequency is as low as 2 through 7 MHz. Hence, a damping factor of ultrasonic waves traveling through steel is low. In terms of roughness of a billet surface and productivity, the time required to pass a steel product through a flaw detector must be minimized. For these reasons, only imperfections of tens of millimeters in length (and hundreds of microns in width) can be detected.
Progress toward increasing load and contact pressure has recently been made in steelworking facilities, and demand for higher temperatures in the field of paper machinery has recently become stricter. For these reasons, a usage environment of a rolling bearing to be incorporated into the facilities has become harsher.
In relation to a rolling bearing used in steelworking facilities and paper machine facilities, there exists demand for a longer life and elimination of products which suddenly become short-lived or which suffer occurrence of cracking. In a production framework in which lines are inspected and maintained at given time intervals, if a rolling bearing used in the line has become short-lived or susceptible to cracking, the line must be deactivated, thereby inflicting an enormous loss. Therefore, there is a need for a rolling bearing which does not become short-lived at high contact pressure and under heavy fitting stress and which obviates a concern about occurrence of cracking and has a stable, long life.
Even in connection with a bearing for use in a railcar, such as the Shinkansen, which is to be continuously used for a long period of time at high speed, a user environment of a bearing has recently become more harsh. In association with an increase in the speed of the Shinkansen, a rolling bearing is exposed to fatigue that is higher in cycles than that having arisen in the related art. Journal of Japanese Society of Tripologists Vol. 45, No. 7 (2000), pg. 27, states that a maintenance-free Shinkansen has recently been planned. In view of facilitating business management of a railroad company, and that there is a desire for increasing an interval for inspection and replacement of a bearing from a current level of 0.9 million kilometers to 1.2 million kilometers and for a maintenance-free bearing of 1.2 million kilometers performance. In these applications, there exists a growing demand for an improvement in reliability of a rolling bearing.
A bearing used in such a train is an important safety component. Occurrence of flaking or cracking in the bearing during operation of a train can lead to a serious accident. Hence, there exists demand for a bearing which is ensured of having a long life and obviates a concern about occurrence of a short-lived bearing even when a bearing has been continuously used over a long period of time at high speed and is susceptible to high-cycle fatigue.
As mentioned previously, with a view toward prolonging life of a bearing in a good lubrication environment, several method have been adopted. That is, a method of limiting the amount of oxygen contained in steel for diminishing non-metallic intervening material, or a method of limiting the amount or size of intervening material present in a given area of object for inspection has been adopted, to thereby extend the life of a bearing. The life of a bearing, as a whole, has been extended, but occurrence of a short-lived bearing has been observed in most cases.
The reason for this is that the related art has not guaranteed absence of intervening material in a raceway ring itself but has limited a typical value of steel to be used. Macro-streak-flaws that suddenly arise within the range of variations in steel have not been specified. When such macro-streak-flaws are present in vicinity of a raceway surface, an unplanned short-lived product is considered to have arisen.
By virtue of considerable study that has been conducted by the present inventors on the rolling bearing set forth, the inventors have found that a short life is ascribable to non-metallic intervening material (imperfections) of tens to hundreds of micrometers located in positions immediately below the bearing raceway surface. Japanese Patent Application Laid-Open No. 130447/2000 has proposed that the fatigue life of the roller is extended by limiting, to 500 xcexcm or less, the length of non-metallic intervening material existing at depths up to a depth of 2% Da [Da denotes a diameter of a rolling element (in the case of a tapered roller, a mean value obtained by division, by 2, of a sum of a smaller diameter and a larger diameter)], thereby extending rolling fatigue life and obviating a concern about occurrence of a short-lived bearing.
The technique disclosed in Japanese Patent Application Laid-Open No. 130447/2000 is found to prevent shortening of the life of a bearing. As found in the course of considerable studies undertaken by the present inventors, it turns out that, if imperfections, such as large non-metallic intervening material, are present in a position deeper than the bounds specified by the above-described patent, cracking may originate from the imperfections in a case where a bearing is used in a paper machine with, e.g., an inner ring, being subjected to fitting stress.
Continued study by the present inventors reveals that, even in a situation in which a bearing is used under less severe load conditions, the bearing maybe short-lived and susceptible to flaking even in the case of presence of intervening material having a length of 500 xcexcm or less, if the bearing is used under high-cycle repeated stress. Simultaneously, the inventors have found that limitation of width and length of non-metallic intervening material located within a depth of 2% Da (Da denotes roller diameter) below a bearing raceway surface prolongs the life of a bearing, thus leading to attainment of the present invention.
As a method aimed at inspecting the distribution of comparatively-small non-metallic intervening material in steel, Japanese Patent Application Laid-Open No. 257761/1997 describes a method of detecting small non-metallic intervening material of around 10 micrometers by utilization of high frequency during a billet phase. The method is to regulate the roughness of a sample steel product by means of grinding the surface of the sample, and to inspect an area on the order of, at most, hundreds of square millimeters. Practical inspection of all steel products is difficult.
In a rolling bearing, a raceway surface is repeatedly subjected to rolling fatigue. As has been well known, presence of large non-metallic intervening material in a bearing (particularly in the vicinity of a raceway surface) exerts a great influence on the life of the bearing. For this reason, in order to extend the life of a bearing, there exists a demand for a method of enabling detection of large non-metallic intervening material in all bearing products beforehand. As a method of solving the problem, the present inventors have proposed, as described in Japanese Patent Application Laid-Open No. 337539/1999, a method of detecting large intervening material in all bearing products by means of inspecting a raceway surface of each bearing ring through ultrasonic flaw detection.
More specifically, the invention has been conceived for the purpose of detecting interior large intervening material from a bearing raceway surface and providing a stable and long-life bearing by means of, in combination, detecting flaws to at least a depth of 2 mm below the surface deeper than a position of maximum shearing stress for a bearing by means of an angle beam method, and detecting flaws in bounds deeper than the range covered by the angle beam method by means of the normal beam method.
The present inventors have conducted considerable study on the form of presence of large intervening material in steel detected by the flaw detection method and detection intensity. The inventors have found that some large pieces of intervening material deteriorate detection sensitivity under the conventional method.
As mentioned previously, the method of detecting non-metallic intervening material has been put forward as described in Japanese Patent Application Laid-Open NO. 337530/1999. As a result of considerable study on an improvement in the sensitivity of detection of intervening material whose size is limited by the invention, the inventors have found that imperfections of tens of micrometers can be detected accurately within all cross sections of a bearing by limiting surface roughness of a bearing raceway surface (for increasing a signal-to-noise ratio of an ultrasonic echo).
The present invention has been conceived on the basis of the drawbacks and findings described above. The present invention aims at providing a rolling bearing, a bearing, and a bearing raceway ring ultrasonic flaw detection method. More specifically, there is provided a rolling bearing that does not become short-lived and obviates a concern about occurrence of cracking even when used in a harsh/severe environment, such as under heavy load and high contact pressure as in the case of a bearing for use in steelworking or at high temperatures with an inner ring thereof being subjected to fitting stress as in the case of a paper machine bearing. There is also provided a bearing which is to be used in an environment, in which the bearing is subjected to high-cycle rolling fatigue stemming from high-speed rotation, as in the case of a bearing used for a railcar such as the Shinkansen or in a machine tool, and which is assured of having no large flaws typified by non-metallic intervening material in the entire volume of a bearing in the vicinity of a raceway surface and a long life without involving a concern about occurrence of a short-lived bearing or a cracked bearing. There is further provided a bearing raceway ring ultrasonic flaw detection method for use with the rolling bearing and with the bearing set forth.
To solve the problems, the present invention provides a rolling bearing, wherein a bearing raceway ring of at least one of inner and outer rings has a surface (such as a raceway surface) which has a roughness of 0.4 xcexcmRa or less and is to act as a surface under ultrasonic inspection, and wherein an imperfection existing in a controlled volume including the axis of rotation of the raceway ring and all cross sections parallel to the rotation axis of the raceway ring assumes a length of 0.2 mm or less as represented by the square root of area of the imperfection.
Here, the term xe2x80x9ccontrolled volumexe2x80x9d designates a volume obtained by means of rotating a hatched cross section X shown in FIG. 1 in the direction of circumference of the raceway ring. In the following description, the cross section will be sometimes called all cross sections located below a raceway surface.
The surface under ultrasonic inspection is a raceway surface, normally.
Imperfections or flaws refer to non-metallic intervening material or an aggregate consisting of pieces of non-metallic intervening material.
A steel bearing typified by a roll-neck bearing is used under heavy load and at high contact pressure. Progress has recently been made on miniaturization of rolling mill facilities, and advancement is also made on miniaturization of a housing having a bearing inserted therein. For example, if a housing becomes deficient in rigidity, there may arise a case where an outer ring is subjected to repeated bending stress as a result of the bearing following the housing.
An inner ring of a paper machine bearing is used while subjected to fitting stress. In a recent ever-increasing high-temperature environment, large hoop stress is exerted on the inner ring of the bearing. There arises a case where cracking originates in large non-metallic intervening material located in the vicinity of an inner-diameter surface.
A result of study on this problem conducted by the present inventors shows that, even in the case of imperfections (including an aggregate of non-metallic intervening material pieces, and the same applies to other, corresponding cases) existing in a controlled volume of all cross sections located below a raceway surface having a length of 0.5 mm or more, if the imperfections assume a length of 0.2 mm or less as represented by the square root of areas of the imperfections, there are prevented shortening of life of the bearing and occurrence of cracking, which would otherwise originate from the imperfections.
Here, the xe2x80x9clength as represented by the root square of area of an imperfectionxe2x80x9d means a value represented by the xe2x80x9csquare root of a product of length and width of an imperfection within a plane located in a position where the imperfection is of maximum size.xe2x80x9d Attention has been drawn to the largest piece of intervening material existing in all cross sections. Origination of cracking is found to be attributable to a combination of the length of an imperfection and a given width or more thereof. For this reason, the size of an imperfection is specified as a length as represented by the root square of area of the imperfection. The present invention specifies the maximum length represented by the root square of area of the imperfection as being a value of 0.2 mm or less.
If hoop stress is exerted on a bearing; e.g., an inner ring, maximum tensile stress develops in a raceway surface. Here, the tensile stress diminishes further toward the inside thereof (i.e., in the direction of outside diameter). If large intervening material is present at any point along the direction of tensile stress, the stress concentrating at the intervening material becomes greater, thereby resulting in occurrence of cracking.
In this regard, the result of the study conducted by the present inventors shows that, when all cross sections located below a surface of a raceway ring under ultrasonic inspection are taken as a controlled volume, there can be prevented occurrence of cracking, which would otherwise originate from imperfections across all the cross sections of the bearing including surroundings of a surface, by means of setting the size of an imperfection (including an aggregate of non-metallic intervening material pieces) to a length of 0.2 mm or less as represented by the square root of area of the imperfection.
As mentioned above, in relation to a bearing to be used in a harsh environment, such as under high pressure and high temperature, the size of an imperfection existing in the controlled volume including an axis of rotation of the raceway ring and all cross sections parallel to the rotation axis is set to a length of 0.2 mm or less as represented by the square root of area of the imperfection.
Imperfections described in connection with the present invention refer to imperfections (including intervening material, macro-streak-flow, void, crack, and so on) of various shapes, such as linear imperfections, oval imperfections, or spherical imperfections. An imperfection echo serving as an imperfection signal is emitted to the area of an imperfection. The area of an imperfection echo is transformed into the area of an imperfection, and the size of the imperfection is defined as a length as represented by the root square of area of the imperfection.
Note that in the specification, when the area of the imperfection is a linear or rectangular shape, the length represented by the root square of area of the imperfection is directed to a length represent by a root square of area defined by multiplying a longitudinal length by a lateral length. Further, in the specification, when the area of the imperfection is an ellipse or circle shape, the length represented by the root square of area of the imperfection is directed to a length represent by a root square of area defined by multiplying the longest diameter by the shortest diameter.
Japanese Patent Application Laid-Open No. 337530/1999 describes a method of detecting non-metallic intervening material of tens to hundreds of micrometers by taking, as a controlled volume, a cross section including a deep location below a raceway surface of a raceway ring. A result of further continued study on enhancement of detection accuracy of ultrasonic flaw detection shows that an imperfection of tens of micrometers over all cross sections of the bearing can be detected accurately by means of limiting surface roughness of the bearing raceway surface.
More specifically, this flaw detection method was proposed as described in Japanese Patent Application Laid-Open No. 337530/1999. In this patent, a bearing which has been ground after heating is taken as favorable. However, a further study on the method shows presence of a range of surface roughness suitable for detection.
A result of a survey on the relationship between surface roughness and detection intensity shows that an ultrasonic flaw detection signal assumes a superior signal-to-noise ratio with regard to intensity at a surface roughness of 0.4 xcexcmRa or less. Detection intensity is found to be deteriorated at surface roughness greater than this, thereby posing difficulty in detecting imperfections of the sizes specified by the invention.
For these reasons, the roughness of a surface under ultrasonic inspection (usually a raceway surface) to be subjected to ultrasonic flaw detection is specified as being a value of 0.4 xcexcmRa or less.
According to the invention, there is also provided a rolling bearing, wherein a raceway ring of at least one of inner and outer rings has a surface (such as a raceway surface) which has a roughness of 0.4 xcexcmRa or less and is to act as a surface under ultrasonic inspection, and wherein an imperfection existing in a controlled volume which is a product of (a 2% Da depth of the surface under ultrasonic inspection) and (the surface under ultrasonic inspection) assumes a length of 0.2 mm or less as represented by the square root of area of the imperfection and/or is smaller than {square root over ((area))}MAX or less.
{square root over ((area))}MAX=[(1.56xc2x7Hv+187)xc3x97(0.77/(xcfx84st)MAX)]6xe2x80x83xe2x80x83(1)
where (xcfx84st)MAX is the maximum shearing stress induced by rolling load on the surface (such as the raceway surface), and Hv is Vickers hardness in the position of the maximum shearing stress beneath the surface.
A length as represented by the root square of area of an imperfection means the root square of an imperfection area.
Further, the term xe2x80x9c2% Da depthxe2x80x9d means a 2% Da depth immediately below a bearing raceway surface. Da means a diameter of a rolling element (in the case of a tapered roller, a mean value obtained by division, by 2, of a sum of a smaller diameter and a larger diameter).
A surface under ultrasonic inspection usually means a raceway surface.
A position of maximum shearing stress is a 2% Da depth from the raceway surface.
The area defined by xe2x80x9ca product of (a 2% Da depth of the surface under ultrasonic inspection) and (the surface under ultrasonic inspection)xe2x80x9d corresponds to, e.g., an area designated by Y shown in FIG. 2.
In connection with a bearing for use in the Shinkansen, as a result of a recent increase in the speed of the Shinkansen (bullet train), the operation speed of some trains has reached a maximum of 300 km/hr. An increase in the operation speed of trains involves further weight reduction of railcars. With regard to bearings, there have hitherto been employed a cylindrical rolling bearing subjected to radial load and a ball bearing subjected to irregular thrust load. In contrast, a conical rolling bearing capable of being simultaneously subjected to radial load and thrust load has recently been employed [see Journal of Japanese Society of Tripologists Vol. 45, No. 7 (2000), pg. 27].
For these reasons, working conditions for a bearing are considered to have become more harsh than those for conventional bearings.
A result of the study conducted by the present inventors shows that a bearing will be subjected to flaking and become short-lived in an environment in which the bearing is susceptible to high-cycle rolling fatigue at high speed, when non-metallic intervening material existing in a controlled volume which is a product of (a 2% Da depth of the surface under ultrasonic inspection) and (a raceway surface); that is, a controlled volume which is a product of (a 2% Da depth of the surface under ultrasonic inspection) and (the surface under ultrasonic inspection), becomes greater than a given value. Determination of a critical size of intervening material has lead to the present invention.
When working conditions have reached a certain level or higher; that is, when intervening material is smaller than a length of 0.2 mm or less as represented by the square root of area of the intervening material, the critical value differs according to the working conditions. Exfoliation is found to arise under conditions in which the size of intervening material exceeds a critical value {square root over ((area))}MAX(=[(1.56xc2x7Hv+187)xc3x97(0.77/(xcfx84st)MAX)]6, where (xcfx84st)MAX is the maximum shearing stress imposed on a bearing and a rolling element.
In other words, under less harsh working conditions, no flaking arises if the size of intervening material is smaller than a length of 0.2 mm or less as represented by the square root of area of the intervening material. The invention according to second aspect of the present invention has been conceived on the basis of this finding.
If working conditions become harsher, flaking will arise even in the case of a length of 0.2 mm or less in terms of square root of area of an imperfection. As mentioned above, the critical value is expressed by {square root over ((area))}MAX=[(1.56xc2x7Hv+187)xc3x97(0.77/(xcfx84st)MAX)]6. An invention according to third aspect of the present invention has been conceived on the basis of the critical value.
An idea presumed from the expression is described as follows.
A quantitative relationship between the size of intervening material and rolling fatigue strength has not yet been obtained. There have already been obtained a bearing steel strength of xcex94K1th=5.8 MPa(m) [The Society of Materials, Japan, Fatigue Department, The 25th Fatigue Symposium Collection, Yoshio FUJII et al., pg. 29, (2000)]; and a bearing steel strength of xcex94K2th=14 MPa(m) [The Society of Materials, Japan, Fracture Mechanics Department, The 14th Tripology Sub-Committee, Yoshinobu MURAKAMI et al., pg. 55, (2000)]. Provided that xcex94K1th less than xcex94K2th, when flaking originates in non-metallic intervening material, fatigue cracks are considered to originate from non-metallic intervening material in K1 mode. In connection with K1 mode, as described in xe2x80x9cInfluence of Micro-Imperfection Debrisxe2x80x9d (Yoshinobu MURAKAMI, Yokendo (Corporation), Mar. 8, 1993), Eq. (2) provided below is deduced from many data sets as an equation for determining a fatigue limit of 107 cycles for internally-originated imperfections.
"sgr"w=1.56(Hv+120)/{{square root over ((area)1/6)}}xe2x80x83xe2x80x83(2)
Here, (area) means a length as represented by the root square of area of an imperfection, and Hv means the hardness of a point of origin.
Here, xcex94K1th and xcex94K2th each represent a threshold stress intensity factor and mean that, if fatigue cracks have become greater in K1 and K2 modes, the cracks will enter a stable growth phase. Here, the unit is MPam.
Here, m means the half power of meter (=m1/2)
Here, K1 mode means a coefficient showing the strength of stress field in the vicinity of the extremity of a crack, and cracks in K1 mode exhibit a style of pulled deformation.
K1 is known as a critical stress expansion coefficient for cracks of open type, and K2 mode is known as a critical stress expansion coefficient for cracks of sheared type.
The above concept is based on the finding that a fatigue limit of material is dependent on the size of intervening material from which cracks originate and the hardness of the intervening material. A result of the life test conducted by the present inventors under various conditions (according to the size of intervening material pieces found through ultrasonic flaw detection and variations in test load) on the basis of Eq. (2) shows that, in connection with flaking of a bearing, there exists a correlation between the size of intervening material and the maximum shearing stress developing between a rolling element and inner and outer rings at the time of use.
If intervening material of (area)MAX is present under the conditions for Eq. (1), flaking will arise. This has been found in applicable to a case where intervening material measures 0.2 mm or more and flaking will be induced by even small shearing stress.
As mentioned above, the method of detecting the imperfections has been proposed as described in Japanese Patent Application Laid-Open No. 337530/1999. In this case, a bearing which has been ground after having been thermally treated is considered preferable. However, the continued research conducted thereafter shows that there are bounds for surface roughness suitable for detection. A result of a survey on the relationship between surface roughness and detection intensity shows that an ultrasonic flaw detection signal assumes a superior signal-to-noise ratio with regard to intensity at a surface roughness of 0.4 xcexcmRa or less. Detection intensity is found to deteriorate at surface roughness greater than this, thereby posing difficulty in detecting imperfections of the sizes specified by the invention. For these reasons, surface roughness is specified as being the above-described value.
A bearing raceway ring ultrasonic imperfection (such as flaw) detection method, in which ultrasonic waves are emitted from a probe to a surface to serve as a surface under ultrasonic inspection of a bearing raceway ring, and imperfections are detected by means of reflection of an echo, wherein
flaws or imperfections are detected to a depth of 2 mm below at least a position of the maximum shearing stress on the bearing raceway ring by means of at least one of the surface beam detection method and ultrasonic flaw detection method.
In the above-mentioned method, a flat beam probe of non-focus type may be used for at least one method.
In the above-mentioned method, it is preferable to further comprises the step of detecting a bearing raceway ring as a non-defecting one if all of imperfection of the bearing has a lenth of 0.2 mm or less as represented by the square root of area of the imperfection.
An invention described in fifth aspect of the present invention provides a rolling bearing, wherein a raceway ring of at least one of inner and outer rings has a surface which has a roughness of 0.4 xcexcmRa or less and is to act as a surface under ultrasonic inspection, and wherein each of the imperfections detected by the ultrasonic flaw detection method described in fourth aspect of the present invention assumes a length of 0.2 mm or less as represented by the square root of area of the imperfection.
According to the invention described in fifth aspect of the present invention, flaw detection methods matching the shape of large intervening material which is an object of inspection are selected or used in combination. As a result, detection of intervening material of any shape or form is possible. According to the invention defined in fifth aspect of the present invention, there can be provided a rolling bearing using a raceway ring, in which the size of contained intervening material is suppressed to a desired size without fail by means of the detection method.
As mentioned previously, as described in Japanese Patent Application Laid-Open No. 337530/1999, the present inventors have proposed a bearing ring ultrasonic flaw detection method for ensuring a stable, long-life bearing by means of detecting and removing large intervening material located immediately below a rolling bearing raceway surface through ultrasonic flaw detection of all bearings. However, a result of continued study shows that intervening material of, e.g., several millimeters in length having a small width, sometimes fails to be detected by the angle beam method using a conventional probe of point focus type. Use of a probe of 2 to 15 MHz flat beam type (non-focus type) is found to enable detection of the intervening material of such shape and form with a superior signal-to-noise ratio. Thus has been conceived the present invention.
The reason why the probe of point focus type fails to detect intervening material is considered to be as follows. A probe of point focus type concentrates ultrasonic energy at a focal point, thus exhibiting a great detection ability. However, the diameter of the focal point measures hundreds of microns or thereabouts. At the time of flaw detection operation, the focal point of energy moves in the vertical and horizontal directions at pitches smaller than the diameter of the focal point, thus detecting flaws or imperfections within a target area. Consequently, detection of a large imperfection is determined by means of detecting flaws or imperfections on a focal-point-by-focal-point basis and linking together the thus-detected points to determine the overall size and length of the imperfection. For example, if there is large intervening material having a width of 50 microns and a length of 5 mm, a probe of point focus type detects the maximum intervening material piece as having a width of 50 microns and a length corresponding to the width of the beam spot (about 0.5 mm). Consequently, the intensity of imperfection echo at each focal point is considered not to become greater.
In contrast, a probe of flat beam type has a minimum of beam diameter of 5 mm or more. In the case of an imperfection which is an object of detection in the present invention, the entirety of the imperfection falls within the diameter of a single beam spot. A resultant echo has comparatively high intensity, and detection of the imperfection is feasible.
The probe of flat beam type is found to have a higher capability to detect elongated intervening material than does the probe of point focus type. The problem can be solved by use of the probe of flat beam type in conjunction with a surface flaw detection method; that is, at least one of the angle beam method and the surface wave flaw detection method. Thus has been conceived the present invention.
As described in Japanese Patent Application Laid-Open No. 337530/1999, a flaw detection frequency ranging from 2 MHz through 30 MHz has been put forward. In general, as a frequency becomes higher, the minimum detectable size of an imperfection becomes smaller. Hence, a higher frequency is beneficial for detecting large intervening material. In a case where the surface wave flaw detection method is employed, a signal-to-noise ratio of an imperfection is admitted to have a tendency of becoming deteriorated with an increase in frequency. From this, a range of 2 MHz to 10 MHz is found to be suitable. In relation to the invention defined in fourth aspect of the present invention, the flaw detection frequency is preferably set to a range of 2 MHz to 10 MHz, in view of enhancement of detection sensitivity.
Moreover, the above-mentioned object can also be achieved by a bearing raceway ring ultrasonic imperfection detection method for a rolling bearing that comprises a bearing raceway ring having a raceway surface which has a roughness of 0.4 xcexcmRa or less, the method comprising:
detecting a bearing raceway ring as a non-defecting one if all of imperfection of the bearing raceway ring has a length of 0.2 mm or less as represented by the square root of area of the imperfection.